1. Field of the Invention
The present invention relates generally to mercaptofunctional high dipole moment and hyperpolarizability (μβ) electro-optic (EO) chromophores and EO polymers, and particularly to mercaptofunctional high μβ EO chromophores and EO polymers useful for making electro-optical devices and systems.
2. Technical Background
Organic chromophores are a basic element needed to build organic polymeric electro-optic materials. Traditionally, organic polymeric EO materials are made via the polymerization of EO chromophores which usually include dihydroxyl functional groups. EO materials are useful in systems and devices for emitting, modulating, transmitting or sensing light. EO chromophores bearing amino, vinyl and carboxylic acid functional groups have also been used in synthesizing organic polymeric EO materials. Organic polymeric EO materials such as polyesters, polyimides and polyurethanes have been synthesized from these monomers.
However, materials such as polyesters, polyimides and polyurethanes have several disadvantages. For example, in order to synthesize some of these polymers, a harsh reaction environment such as acidic or basic reaction conditions are needed; and in addition high temperatures are needed for the polymerization reaction to occur. These harsh polymerization conditions usually result in the destruction of the EO chromophores. Another disadvantage of EO chromophores bearing hydroxyl or amino functional groups is that these chromophores usually cause a large optical loss at major communication wavelengths, for example at 1550 nm.
Mercaptofunctional high μβ EO chromophores are especially sensitive to harsh polymerization conditions and are not stable to basic, acidic and free radical polymerization conditions. However, mercaptofunctional high μβ EO chromophores are advantageous for use in organic polymeric EO materials due to the low optical loss associated with —SH functional groups on the chromophore. The incorporation of these —SH chromophores into electro-optical polymers has proven challenging due to the sensitivity of the chromophores' terminal —SH groups. Several years have been spent trying to determine alternative polymerization reactions that can be carried out under mild polymerization conditions to produce organic polymeric EO materials having high glass transition (Tg) temperatures with low optical loss at communication wavelengths.
Organic polymeric EO materials having the following properties would be particularly useful. First, the active component, the EO chromophore, should be thermally and photochemically stable. Second, the EO chromophore should have a high molecular nonlinearity while being compatible with the polymer host material. Third, both the EO chromophore and the polymer host materials should possess optical transparency at communication wavelengths. Fourth, the EO chromophore should be able to be aligned in a noncentersymmentric fashion by poling and should remain stable for many years while retaining a high EO coefficient.
Bismaleimides are known as high temperature thermosetting resins having high Tg temperatures (some greater than 200° C.). These thermosetting resins cure via addition polymerization to produce high performance polymeric materials. Bismaleimides can be reacted with diamines or dithiols forming Michael type adducts, polyimides, or poly(imido sulfides) through a step-growth polymerization mechanism. Although some complicated side reactions may occur in the reaction between bismaleimide and diamine or dithiols which can lead to the production of insoluble crosslinked polymers, a polymerization solvent having an acidic hydrogen atom can effectively suppress the side reactions from the anionic mechanism to give rise to linear polymers. During the 1980's, the structure and properties of different hydrocarbon poly(imido sulfide) materials were extensively investigated by General Electric and The Dow Chemical Company. However, Michael addition polymerization producing an EO chromophore containing poly(imido sulfide) polymer useful for organic polymeric EO materials and devices has presented challenges due to the harsh polymerization conditions typically required for a covalent bond between the poly(imido sulfide) polymer and the EO chromophore.
Octafluorobenzidine bismaleimide and its Michael adducts with fluorine-containing diamines are used in the preparation of highly fluorinated poly(imido amides). However, poly(imido amides) cannot be used for organic polymeric EO materials and devices at communication wavelengths, because of the strong absorption of NH groups in the polymer chain. Halogen-containing bismaleimide derivatives reacted with a diol are typically used to prepare polyether-polyimides material for the optical communication industry. There is a low glass transition (Tg) disadvantage associated with some halogenated polymers such as halogenated acrylate polymers, in particular, fluorinated polyimides. Another disadvantage is the solvent process capability of fluorinated polyimides.
For the foregoing reasons, the technical challenge for synthesis of a high Tg, low optical loss, covalently bonded and high μβ EO chromophores containing polymers lies not only in the synthesis of the polymeric host material, but also in the polymerization reaction incorporating the EO chromophore into the final material. Since polymerization conditions as discussed above can destroy the unique donor, acceptor and electron rich bridge electronic structures of the high μβ chromophores, especially mercaptofunctional high μβ chromophores whose incorporation in EO materials would be advantageous due to the low optical loss associated with the —SH functional group, improved methods for synthesizing these materials is highly desirable.